1. Field of the Invention
This invention relates to a method of preheating a refractory lined reactor. More particularly, this invention relates to preheating a refractory lined ported rotary kiln which is part of a plant for gasifying coal where the plant includes apparatus fabricated in part from materials susceptible to chloride induced stress corrosion cracking in the presence of oxygen.
2. Description of the Prior Art
Copending patent application of P. G. Garside, Ser. No. 264,479 filed May 18, 1981, discloses a process for gasifying solid carbonaceous material, such as coal, in a refractory lined ported rotary kiln gasifier. According to the disclosed process, coal is admitted to an uphill inlet end of the kiln. The coal forms a bed within the kiln which slowly moves toward a discharge end due to the rotation and inclination of the kiln. As the coal proceeds through the kiln, air and steam are admitted into the kiln through ports to treat the coal and effect conversion of the coal to a combustible fuel gas. The gas so produced in the kiln is removed and further treated to cleanse the gas and recover sensible heat therefrom. Before the gasification process can proceed, the refractory lining is preheated to a temperature profile sufficient to maintain autothermic operation of the process. Autothermic operation may be described as an operation where heat supplied by air and steam admitted through the kiln ports together with heat released through exothermic reactions within the kiln are sufficient to maintain the gasification process.
Carbonaceous material, such as coal, used for feedstock for such a process as described contains numerous impurities. One such impurity is chlorine. For example, in Illinois No. 6 coal, chlorine is present in the coal in amounts of 0.02% to 0.4% by weight. During the gasification process, the chlorine is liberated and flows with the produced gas into the cleansing apparatus.
The cleansing apparatus used in a gasification plant includes apparatus such as cyclone separators, heat exchangers and related piping. Such equipment is commonly fabricated from corrosion resistant material such as austenitic stainless steel. The simultaneous presence of oxygen and chlorine dissolved within gases with this equipment poses the risks of adverse interaction between the chlorine and the stainless steel. Namely, under certain conditions, austenitic stainless steel is subject to stress corrosion cracking when exposed to chlorine. As reported in "The Effect of Chloride and Oxygen on the Stress Corrosion Cracking of Stainless Steels: Review of Literature" by Barry M. Gordon, Materials Performance (April, 1980), chlorine induced stress corrosion cracking of austenitic stainless steels occurs in the presence of oxygen. As indicated in the Gordon article, only small amounts of oxygen need be present to initiate chloride induced stress corrosion cracking.
Commonly, oxygen is admitted to a plant by air entering the plant during shutdown. Certain practices have been recommended to prevent stress corrosion cracking in such circumstances. For example, NACE Standard RP-01-70 entitled "Recommended Practice--Protection of Austenitic Stainless Steel in Refineries Against Stress Corrosion Cracking by Use of Neutralizing Solutions During Shut Down", published by the National Association of Corrosion Engineers (October, 1970), recommends a procedure to clean and purge refinery equipment during shutdown.
Oxygen may also be admitted during the start-up of plants having a reactor which must be heated to operating temperature before the plant may operate. In preheating the reactor, a burner is used to combust air and fuel to produce an exhaust gas which heats the reactor. Oxygen contained within the exhaust gas enters the plant equipment and, if chlorine is also present, poses a danger of stress corrosion cracking within the equipment. The problem is further compounded by the need to heat the reactor at a controlled rate to a desired temperature profile.